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A Nixpkgs module system usage pattern

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# The Dendritic Pattern

A [Nixpkgs module system](https://nix.dev/tutorials/module-system/) usage pattern

## Testimonials

> I adore this idea by @mightyiam of every file is a top-level module and I think I will adopt it everywhere.

—Daniel Firth (modified from [source](https://x.com/locallycompact/status/1909188620038046038))

> Massive, very interesting!

—Pol Dellaiera ([source](https://discourse.nixos.org/t/pattern-every-file-is-a-flake-parts-module/61271/2?u=mightyiam))

> I’ve adopted your method. Really preferring it.

—gerred ([source](https://x.com/devgerred/status/1909206297532117469))

## Background

[NixOS](https://nixos.org/manual/nixos/unstable/),
[home-manager](https://github.com/nix-community/home-manager) and
[nix-darwin](https://github.com/nix-darwin/nix-darwin)
are popular projects that allow the user to produce [derivations](https://nix.dev/tutorials/nix-language.html#derivations)
that can be customized by evaluating a [Nixpkgs module system](https://nix.dev/tutorials/module-system/) configuration.

Figuring out a practical and expressive architecture for a codebase that provides configurations had proven to cost many a Nix user numerous rounds of re-architecturing.

Factors contributing to the complexity of the matter:

- Multiple configurations
- Sharing of modules across configurations
- Multiple configuration classes (`nixos`, `home-manager`, etc.)
- Configuration nesting such as home-manager [within NixOS](https://nix-community.github.io/home-manager/index.xhtml#sec-install-nixos-module) or [within nix-darwin](https://nix-community.github.io/home-manager/index.xhtml#sec-install-nix-darwin-module)
- Cross-cutting concerns that span multiple configuration classes
- Accessing values such as functions, constants and packages across files

## Required skills

Understanding the dendritic pattern strictly depends on fair comprehension of the following:

- [Nix language](https://nix.dev/tutorials/nix-language)
- [Nixpkgs module system](https://nix.dev/tutorials/module-system/)
- [The `deferredModule` type](https://nixos.org/manual/nixos/stable/#sec-option-types-submodule)

## The pattern

### Top-level and lower-level modules and configurations

The dendritic pattern reconciles these factors using yet another application of the Nixpkgs module system—a *top-level configuration*.
The top-level configuration facilitates the declaration and evaluation of lower-level configurations, such as NixOS, home-manager and nix-darwin.

Commonly, this top-level configuration is a [flake-parts](https://flake.parts) configuration,
but it does not have to be.
Alternatives to flake-parts may exist.
Also, the module system can be used directly via (`lib.evalModules`).

In the dendritic pattern every Nix file
except for entry points such as `default.nix` and `flake.nix`
is a module of the top-level configuration.
In other words, every Nix file that isn't an entry point is
a Nixpkgs module system module that is imported directly into the evaluation of the top-level configuration.
Additionally, every top-level module:

- implements a single feature
- ...across all configurations that that feature applies to
- is at a path that serves to name that feature

Lower-level modules and configurations
such as NixOS, home-manager and nix-darwin
are stored as option values in the top-level configuration.

Lower-level modules take part in the evaluation of any number of lower-level configurations.

### Lower-level module merging

The Nixpkgs module system type `deferredModule` and similar implementations feature _value merging_.
This allows multiple lower-level module values to merge under one distinct name.

For example, in a top-level module, a lower-level module can be declared:

```nix
# modules/audio.nix
nixos.modules.audio = { /* content */ };
```

Or it may be merged with others under a common name:

```nix
# modules/audio.nix
nixos.modules.pc = { /* content */ };
```

## Benefits

### Type of every file is known

The common question "what's in that Nix file?" is made irrelevant.
Non-entry point files contain a Nixpkgs module system module
of the same [`class`](https://nixos.org/manual/nixpkgs/stable/#module-system-lib-evalModules-param-class)
as the top-level configuration.

### Automatic importing

Since all non-entry-point files are top-level modules
and their paths convey meaning only to the author,
they can all be automatically imported using a trivial expression
or [a small library](https://github.com/vic/import-tree).

### File path independence

In some patterns file paths are significant to some particular detail,
such as the type of the expression the file contains
or what specific configuration it belongs to.

Contrary to those, in this pattern a file path represent a feature.
Each file can be freely renamed and moved,
and it can be split when it grows too large or too complex.

## Real examples

- [Shahar "Dawn" Or (@mightyiam)](https://github.com/mightyiam/infra) ([adoption commit](https://github.com/mightyiam/infra/commit/b45e9e13759017fe18950ccc3b6deee2347e9175))

Other examples, for which the author cannot vouch:

- [Victor Borja (@vic)](https://github.com/vic/vix) ([adoption pull request](https://github.com/vic/vix/pull/115)) ([forum answer](https://discourse.nixos.org/t/how-do-you-structure-your-nixos-configs/65851/8))
- [Pol Dellaiera](https://github.com/drupol/nixos-x260) ([adoption pull request](https://github.com/drupol/nixos-x260/pull/83)) ([blog post](https://not-a-number.io/2025/refactoring-my-infrastructure-as-code-configurations/))
- [Horizon Haskell](https://gitlab.horizon-haskell.net/nix/gitlab-ci)
- [Gaétan Lepage](https://github.com/GaetanLepage/nix-config) ([acknowledgment commit](https://github.com/GaetanLepage/nix-config/commit/3ed89eae1a8e13c1910eac5f89f2cdb4f48756ff))
- [Michael Belsanti](https://tangled.org/quasigod.xyz/nixconfig)
- [Oliver Davies](https://git.oliverdavies.uk/opdavies/nix-config)

## Community

- [GitHub Discussions](https://github.com/mightyiam/dendritic/discussions)

- [Matrix room: `#dendritic:matrix.org`](https://matrix.to/#/#dendritic:matrix.org)

## Anti-patterns

### Not declaring options

Using _only_ existing options (such as flake-parts' `flake.modules`)
for the storage of lower-lever modules prevents us from translating our mental model of the system into code.
Declaring options for the storage of modules is easy:

```nix
# modules/nixos/base.nix
{lib, ...}: {
  options.nixos.base = lib.mkOption {
    type = lib.types.deferredModule;
  };
}
```

Using them in another top-level module is also easy:

```nix
# modules/nixos/pc.nix
{config, lib, ...}: {
  options.nixos.pc = lib.mkOption {
    type = lib.types.deferredModule;
  };
  config.nixos.pc = config.nixos.base;
}
```

So, go ahead and model your infra the way serves you best.

### `specialArgs` pass-thru

In a non-dendritic pattern some Nix files may be modules that are lower-level
(such as NixOS or home-manager).
Often they require access to values that are defined outside of their config evaluation.
Those values are often passed through to such evaluations
via the `specialArgs` argument of `lib.evalModules` wrappers like `lib.nixosSystem`.

For example, `scripts/foo.nix` defines a script called `script-foo`
which is then included in `environment.systemPackages` in `nixos/laptop.nix`.
`script-foo` is made available in `nixos/laptop.nix` by injecting it
(or a superset of it, such as the flake `self` may be) via `specialArgs`.
This might occur even once deeper from the NixOS evaluation into a nested home-manager evaluation
(this time via `extraSpecialArgs`).

In the dendritic pattern
every file is a top-level module and can therefore add values to the top-level `config`.
In turn, every file can also read from the top-level `config`.
This makes the sharing of values between files seem trivial in comparison.

### Lower-level module name proliferation

One may be tempted to assign each lower-level module to its own unique name.
Such granularity would result in a great number of named modules.
The cost of such a pattern is that `imports` lists would end up being much longer than necessary.
For example, a `nixos.modules.pc` module would import `with config.nixos.modules; [fonts graphics audio <...and many more>]`.
Another cost is that when a new named lower-level module is added,
it would have to be added to all of the import lists in which it should be.
That goes for the removal of named lower-level modules, as well.

Consider merging multiple non-distinct lower-level modules under one distinct name.

### Fanaticism

The dendritic pattern is merely a pattern.
It's not a religion, law or a mandate.
Use it where it fits and make exceptions where appropriate.
Having Nixpkgs `callPackage` files interspersed among top-level module files
is one reasonable exception to the pattern.
Of course, they'd have to be excluded from automatic importing.
A suggested naming scheme for these: `my-package.pkg.nix`.

### `enable` options

Often created using `lib.mkEnableOption`, `enable` options are necessary when a module is imported even though a feature it provides should not necessarily be enabled.
This anti-pattern originates in NixOS, where most NixOS modules provided by Nixpkgs are imported by default.
That was not a good idea. Don't repeat it.
In most cases, importing a module should enable the feature that it provides.

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